Devices and methods for gauging and measuring manufactured articles for purposes of determining whether they meet quality, dimension and other requirements are well known. However, in view of the present day movement toward increased manufacturing efficiency and speed, there is a need for improved part inspection technology that is well beyond what is presently available. In a machine tool industry investigation which produced the "Report of the Machine Tool Task Force", 1980 Volume 5, Page 82, it is stated that there is a need for "higher speed machines which bring metrology up to machining in terms of inspection time."
The well-known mechanical contacting probe is a current measuring technique which is both slow and relatively inflexible in its application. Of the potential alternative replacement technologies for the mechanical contacting probe, opto-electronic techniques are currently receiving the most attention. The primary reasons for this selection appears to be the advent of economical laser sources and solid state imaging devices and the inherent speed of opto-electronic devices. Furthermore, the various optical techniques are capable of measuring a wide range of materials and can provide high resolution while closely approximating the point measurement capability of mechanical probes.
Some optical/electronic technologies with potential part measurement use include vision approaches such as 2-D cameras and digital image processing, optical image processing, null tracking optical scanners, interferometry, ray optics edge detection scanning, Fourier optics processing, and ray optics triangulation. Current research in vision application areas focuses on part identification, part inspection, and machine guidance. Vision systems utilize relatively large photodiode arrays that provide data requiring a substantial amount of signal processing. Computation intensive algorithms are required and the overall result is that current vision approaches have slow processing speeds and poor range/resolution ratios unless very large photodiode arrays are utilized which, in turn, exacerbates the processing speed problems. In optical imaging systems, the image intensity distribution is focused on an appropriate "matched" filter plate which results in an intensity distribution "product" behind the filter. The amplitude of the final projection "product" represents the amplitude of a cross correlation double integral which can be used to identify the pattern or shape of the image projected onto the filter plate. Optical image processing has the problems of requiring substantial data processing because a whole surface image is produced and limitation to dedicated applications due to the requirement of a holographic plate for each shape to be inspected. Null tracking techniques using optical probes are relatively simple and of fast response within their limited range. However, their dynamic range limitation requires either slowly changing surfaces or slow traverse rates for measurements. Interferometry is very fast and has practically unlimited range for part measurements, however, the technique inherently produces path length errors due to surface roughness of the part being measured. Ray optics edge detection techniques are based on measurement of the shadow produced by part edges when illuminated from a point source. Edge detection is fast, inexpensive, high in both range and resolution and insensitive to surface scatter with proper beam size and signal processing. However, edge detection is limited to only those situations in which the surfaces being inspected produce shadows and therefore lacks the flexibility for general part inspection methods. Recent work in optical Fourier transform techniques for part measurement have been either in optical pattern recognition or in surface roughness measurement. In these efforts, primary emphasis has been placed on higher spatial frequencies more useful in determining part roughness than the geometry of a part. However, with further development, it is anticipated that optic Fourier transform theory will have application to part geometry measurement.
Of the various optical/electronic measurement device alternatives, ray optics triangulation is the most similar to the conventional mechanical contacting probe. In a probe utilizing ray optics triangulation, the location of the intersection of a focused light beam with a part surface is measured. This approach treats light beams as rays, ignoring wave properties of the light, and relies on position detection photosensors. In the mid-1970s, suitable silicon photodetector devices appeared and led to the development of a number of part measurement devices of the optical triangulation type. A relatively early device of this type was a null tracking optical triangulation sensor for in process measurement of turned parts. This device achieved a resolution to 0.0001 inches, but was greatly limited in its usefulness by the null tracking feature. Also at a relatively early time, Diffracto Ltd. developed a more general probe system for use in inspecting turbine blade profiles. The Diffracto device was based on an optical triangulation probe using a linear array of photodiodes. The probe had a limited range of .+-.0.020 inches but had a theoretical resolution of 20 microinches. A serious drawback of the probe was its limited range. The Diffracto probe also included a method of measuring surfaces without maintaining a particular orientation to the surface normal. The system used two symmetrically located photodetector arrays positioned on either side of the incident light beam on the surface so that if the surface inclined toward either of the two photodetector arrays, that array would receive a higher level of light intensity and would be selected as the array to read for measurement purposes. Another optical triangulation probe development, at Case Western Reserve University, scanned the part using a rotating mirror scanner and measured the angular location of the specular peak. This approach was configured to sense only the single peak. The foregoing and other initial efforts in the optical triangulation probe development area utilized a variety of signal processing approaches including analog peak detector networks, digital first moment calculations, and serial digital signal processing.
In all of the currently developed optical measuring probes, there is no provision for both high resolution and acceptable range for high speed part measurement systems. Moreover, speed of data analysis is unsatisfactory, particularly for systems possessing high range-to-resolution ratio. The probe orientations of all currently developed optical probes are limited to near normal and do not provide for probe orientation to accommodate varying inclination of the surface being measured. It is a consequent object of this invention to provide an optical triangulation measuring probe apparatus and a method for optically measuring the inclination and displacement of workpiece or part surfaces having a high range-to-resolution ratio in the measurements, a high speed measurement capability, and flexibility in handling a variety of shapes and parts. It is also an object of the invention to provide a measuring probe apparatus and method in which displacement as well as inclination of a workpiece surface can be measured with the apparatus and as part of the method.